1. Field of the Invention
The present invention relates to an image forming method for visualizing by a reversal development an electrostatic latent image formed through an exposure to a long wavelength light as an image exposing light source such as a laser beam, and more particularly to an image forming method using long wavelength light source for preventing generation of a memory phenomenon on a copy obtained wherein the preceding formed image appears on the next copying cycle due to the generation of excess carriers at the time of the repeated copying operation.
2. Description of the Prior Art
In recent years, printers have been proposed according to various needs for information processing, wherein a long wavelength light source such as semiconductor laser is used as an image exposing source. With this, photosensitive members have been developed which are sensitized in the region of long wavelengths. For example, there has been proposed a photosensitive member wherein amorphous silicon or amorphous silicon: germanium is formed on a substrate by the glow discharge decomposition process, one wherein selenium alloy is vapor-deposited and further another one wherein copper-phthalocyanine or CdS is dispersed in a binder resin for coating on the substrate.
On the other hand, in the printer using the above-mentioned photosensitive member which is sensitized in the region of long wavelengths, the photosensitive member should preferably be irradiated with light of long wavelength similar to the one used for image exposing at the time of erasing the residual potential, as it is best to utilize the inherent sensitivity characteristics of the photosensitive member itself. However, the irradiation for erasing the residual potential with long wavelength light generates charge carriers in the entire photoconductive layer which remain for a limited time until the charging step for the formation of the latent image prevents excess carriers from recombining thereby causing reduction of the charged surface potential.
On the other hand, irradiation for erasing with a small quantity of light in order to prevent the reduction of the surface potential is insufficient to erase the residual potential completely thereby causing a memory phenomenon, wherein the preceding image appears on the following copying cycle. Therefore, the photosensitive member must be irradiated for erasing the residual potential with a large quantity of light as well as be charged with a large charging output in order to remove the memory phenomenon without reducing the surface potential. However, there arises such a drawback that the large charging output hastens deterioration of photosensitive members because of the influence by corona ions on photosensitive members.
The generation of the memory phenomenon will be detailed hereinbelow.
FIG. 1a to FIG. 1e show a conventional image forming method. In FIG. 1a, a photosensitive member 1 comprising a substrate and a photoconductive layer formed thereover is positively charged and then exposed with a laser 2 having long wavelength light source. The exposing portion (image portion) (a) exposed with long wavelength light is light-excited in the photoconductive layer to generate charge carriers, whereas no charge carriers are generated in the non-exposing portion (non-image portion) (b). With the generation of charge carriers in the exposing portion, electrons neutralize positive charges on the surface of the photoconductive layer, whereas holes move toward the substrate. Thus formed electrostatic latent image is visualized with a reversal development as shown in FIG. 1b wherein toner is deposited on the exposing portion (a) of the photoconductive layer.
The developed image is then transferred onto a paper, while the photosensitive member, after having removed the residual toner, is irradiated with long wavelength light 3 in order to erase the residual potential. By this, the photoconductive layer is wholly light-excited, so that electrons neutralize positive charges on the surface, whereas holes move toward the substrate at the non-exposing portion (b). However, at the exposing portion (a), the surface potential has already decreased at the time of image exposing, with the result that the excess carriers incapable of recombining remain in the photoconductive layer. When the photosensitive member is, in this state, charged by a corona charger 4 for carrying out the next copying cycle, the excess carriers remaining in the exposing portion (a) recombine with the charges charged by the charger (4) as shown in FIG. 1d, i.e., electrons recombine with positive charges on the surface of the photosensitive member. On the other hand, no excess carriers are present in the non-exposing portion (b). Consequently, there arises the difference of the surface potential between in the exposing portion (a) and in the non-exposing portion (b) as shown in FIG. 1e, to thereby cause the memory phenomenon.
Moreover, when the photosensitive member is to be charged to a predetermined surface potential in the above-described method, the irradiating light must be set to the value in quantity of preventing the generation of the residual potential. However, this in turn brings about the drawbacks of the requirement to irradiate with a large quantity of light as well as to charge the photosensitive member to a predetermined surface potential with large quantity of electric current. Such drawbacks as mentioned above are unavoidable regardless of the charging polarity.
As described above, printers using in particular the long wavelength light as an image exposing source cause the memory phenomenon to occur during repeated copying cycles, with the result that satisfactory copies cannot be obtained. This fact is remarkably observed in using a photosensitive member sensitized in the region of long wavelength. That is because such kind of photosensitive member is constituted to narrow the optical band gap in order to generate pairs of carriers in the photoconductive layer by low energy light, and as a result, carriers are generated in excess to cause the memory phenomenon. Moreover, in the photosensitive member described above, the irradiating light for preventing the generation of the residual potential and the electric current for charging the photosensitive member to a predetermined surface potential are required to be set to high quantities thereby placing various restrictions on the setting condition for forming images to greatly influence the photosensitive member.